The present invention relates to an apparatus for processing image data read by a scanner or the like from an original document into binary data.
Generally, image data (analog signal) read by a scanner from a document is compared with a threshold level to be converted into binary data. A prior art apparatus designed for this operation fixes the threshold level at a predetermined value when the image on the document is a character, a figure or like black-white image, while rendering the threshold variable in response to a change in the image density level when the image is a photograph or the like which involves halftone. The variable threshold level well reproduces the halftone and a procedure using such a threshold level is usually referred to as the dither method.
With this type of apparatus, the operator enters a threshold level command signal which selects either a fixed threshold level or a dither threshold level. A comparator compares a level of analog image data read by the scanner with the selected threshold level. The output of the comparator will be low or white level when the voltage level of the image data is higher than the threshold level and high or black level when otherwise, whereby the image data is converted into binary data.
The selection of the dither threshold level or the fixed threshold level relies on the operator's decision which is made by observing images page by page. Once the operator specifies the dither threshold level, the whole page will be processed by the dither method resulting in a substantial amount of high or black level data. Then, a facsimile apparatus, for example, will consume a prohibitive period of time for the transmission of such an amount of binary image data after compressing them.